Abstract: Known methods for forming trench storage capacitors require the chemical vapour deposition (CVD) of an undoped silicon oxide layer in order to prevent auto doping of side wall of a semiconductor trench. This layer is deposited once an arsenic doped silicon oxide layer has been disposed and etched to an appropriate depth. Such a technique results in a complex and expensive process. It is therefore proposed to deposit (step 906) the undoped silicon oxide layer 108 in-situ immediately after the arsenic doped silicon oxide layer 106 has been deposited (step 904) and before etching takes place (step 910). It is thus possible to remove the CVD of the undoped silicon oxide, thereby simplifying the overall process and yielding a device having improved performance characteristics.